Bafang CST – Oil Cooling

I got a ‘like new’, used 20″ Bafang CST geared hub motor from Ebay. The seller stated it only ran 170km. Definitely no exaggeration, since it still turned with noticeably resistance. The gears and freewheel clutch weren’t even proper run-in yet.

After a short test run with the Phaserunner and its setup software, I could determine the exact motor characteristics (8.83 rpm/V).
Thats similar to the Bafang BPM with motor code 10 and the exact Bafang CST version I was looking for. All vendors I checked over the last couple of years, unfortunately only sell 26″ and 28″/700 versions of the CST.

After that, it got disassembled, cleaned from grease and set-up for steel planetary gears and oil cooling.

Bafang CST_original planetary gears
the opened-up Bafang CST

Unfortunately the freewheel clutch from the Bafang BPM doesn’t fit. Like they say, there is always something…

Bafang CST_original gears
The CST got a similar freewheel clutch as the Bafang BPM, but with one set key instead of the usual four set keys machined into the axle.

Notice the little black steel washer on the freewheel clutch? Thats a spring washer that takes up any axial play of the motor inside the hub.

Bafang CST_magnet ring and sun gear
Instead of the usual three screws for mounting the sun gear to the magnet ring, this CST got six screws. Another nice touch is the already installed O-ring at the motor cover.

Preparation for oil cooling
After taking the motor apart, two of the brake disk holes got drilled through, to allow filling oil into the motor with a syringe. I plan on using about 20 ml of ATF primarily for lubricating the steel planetary gears. The oil will help to shed some heat from the stator coils and magnet ring to the hum motor shell as well, though.

Bafang CST_drilling brake disk hole
drilling with 4.1mm drill

Bafang CST_cutting thread
cutting the M5 thread through

For some reason, this Bafang CST doesn’t look quite like the others. The axle doesn’t have a M12 nut on the brake disk side (like the BPM), but instead a sealing ring and a smooth 15mm axle diameter. Since the usual nut is, despite my sealing attempts, sipping at least a little oil in the oil-cooled Bafang BPM, this setup should keep the motor clean.

Bafang CST_steel planetary gears
ready for reassembling with new steel planetary gears

Bafang CST_ test stand
The CST on my DIY motor test stand. Shame about the chipped paint around the screws. The CST looked really quite pretty before.

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